UPD17708 datasheet(311/356 Pages) NEC

311 / 356 page

311

µPD17704, 17705, 17707, 17708, 17709

Example 1.

MEM

0.01H

; 1st digit of key input data of security code

MEM

0.02H

; 2nd digit of key input data of security code

MEM

0.03H

; 1st digit data when security code is changed

MEM

0.04H

; 2nd digit data when security code is changed

MEM

0.11H

; 1st digit of current security code

MEM

0.12H

; 2nd digit of current security code

START:

Key input processing

← contents of key A

; Security code input wait mode

← contents of key B

; Substitutes contents of pressed key to R1 and R2

SET2

CMP, Z

; <1> ; Compares security code and input data

SUB

R1, M1

SUB

R2, M2

SKT1

ERROR

; Input data differs from security code

MAIN:

Key input processing

← contents of key C

; Security code rewriting mode

← contents of key D

; Substitutes contents of pressed key to R3 and R4

M1, R3

; <2> ; Rewrites security code

M2, R4

; <3>

MAIN

ERROR:

Must not operate

Suppose the security code is “12H” in the program in Example 1. The contents of data memory addresses M1

and M2 are “1H” and “2H”, respectively.

If CE reset is effected, the contents of key input and security code “12H” are compared in <1>. If the two are the

same, the normal processing is performed.

If the security code is changed in the main processing, the new code is written to M1 and M2 in <2> and <3>.

Suppose the security code is changed to “34H”. Then “3H” and “4H” are written to M1 and M2 in <2> and <3>.

If CE reset is effected as soon as <2> has been executed, program execution is started from address 0000H, without

<3> being executed.

Consequently, the security code is set to “32H”, making it impossible to clear the security system.

In this case, create the program shown in Example 2.

UPD17708 Datasheet(PDF) 311 Page - NEC

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